Some jobs just suck and cause enormous amounts of stress all on their own, but if you have a history of feeling stressed at work—regardless of the job—Dr. Fredric Neuman points out that it's probably because you're trying to be perfect and avoid critcism:

Very often, however, people know very well they are in no danger of being fired, yet feel stress on the job anyway, often on a daily basis. It turns out that for most of them there are two underlying fears not immediately apparent that make them susceptible to stress under ordinary circumstances and that would affect them no matter what kind of work they would do:

1. The fear of making a mistake.

2. The fear of being criticized, or yelled at.

They start off feeling some sense of inadequacy, usually for reasons having to do with the way they were treated growing up.

Whether you were subject to high expectations or anything else, you're never going to do anything that doesn't elicit criticism at least some of the time. And criticism is good, when it's constructive. Rather than put the world on your shoulders and expect yourself to be perfect, understand that the criticism often is an effort to make your work even better. You have to let go of those expectations if you want to let go of your stress.